1. Field of the Invention
This invention relates to a method of and apparatus for an computing altimetric compensation value and an ouput compensation value of an air flow meter in an engine having electronic fuel injection control for operating a fuel injection valve according to electric signals to control the fuel supply to an intake system.
2. Description of the Prior Art
The density of air varies with the altitude in which an automobile travels. Also, the output characteristics of an air flow meter for detecting intake air flow are subjected to change over time since the intake air flow leaking from the periphery of a measuring plate swinging in relation to the intake air flow varies with the accumulation of strain on a wall of the air flow meter. Since the supply of fuel from a fuel injection valve in an engine with electronic fuel injection control is computed on the basis of the output of the air flow meter, it is necessary to compute a compensation value for the change in altitude or output characteristics of the air flow meter for compensating the fuel supply on the basis of this compensation value so as to maintain an air-fuel ratio of the mixture in a combustion chamber at a stoichiometric one. Also, in an engine wherein evaporated fuel adsorbed by activated charcoal as adsorbent is purged to the intake system during running of the engine to prevent the evaporated fuel from being purged from a fuel tank to the atmosphere, the air-fuel ratio of the mixture, i.e the output of air-fuel ratio sensor, varies with an amount of purged evaporated fuel in addition to the fuel supplied from the fuel injection valve. In the normal method of computing said compensation value, the feedback air-fuel ratio (the feedback air-fuel ratio represents the actual air-fuel ratio in the combustion chamber of the engine) is calculated on the basis of feedback signals from an air-fuel ratio sensor to adjust said compensation value on the basis of deviation of the feedback air-fuel ratio from the base one. However, when the running of the engine is once stopped and then resumed, it takes a predetermined time for the air-fuel ratio sensor to be properly heated to produce an effective output. Thus, for this predetermined time and during the low temperature of the engine, the feedback signal from the air-fuel ratio sensor is interrupted and an amount of fuel injection is calculated by an open loop control. Also, for this predetermined time and during the low temperature of the engine, the purging of the evaporated fuel to the intake system is interrupted and the amount of fuel injection, ignition timing, etc. is corrected on the basis of the final compensation value in the previous running of the engine so that the correction of the compensation value caused by the purging of the evaporated fuel to the intake system must be avoided.
In such a prior method of computing the compensation value, the running conditions of the engine are divided by the intake air flow into a plurality of ranges, for example, and a complementary RAM (Random Access Memory) is provided for each range. The data stored in the complementary RAM are always preserved in spite of the position of an ignition switch. The compensation values computed in every range on the basis of the deviation of the feedback air-fuel ratio from the base one are stored in a volatile RAM so that the number of the individual complementary RAMs is increased. Also in the prior computing method, the deviation of the feedback air-fuel ratio from the base one is limited in each range and when the deviation exceeds the limit, the compensation value is adjusted by the limit value, considering that the output of the air-fuel ratio sensor is affected by the purging of the evaporated fuel to the intake system. When an automobile travels from a low to a high altitude with high temperature and high speed (conditions in which a great amount of evaporated fuel is produced from a fuel tank), the deviation always exceeds the limit so that the change in the feedback air-fuel ratio caused by the change in the atmosphere is neglected and no altimetric compensation can be carried out by the prior method. Or, in another prior method of performing the altimetric compensation, a path of evaporated fuel purged to the intake system is closed by an electromagnetic valve to provide a purge interrupting period of the evaporated fuel for correcting the compensation value during this period. However, the accuracy of the air-fuel ratio is degraded since the frequency of correction of the compensation value during this period is reduced.